1. Field of the Invention
This invention relates to a liquid crystal alignment agent, more particularly to a liquid crystal alignment agent having improved pretilt angle uniformity and process stability. The invention also relates to a liquid crystal alignment film formed from the liquid crystal alignment agent, and a liquid crystal display element including the liquid crystal alignment film.
2. Description of the Related Art
Nematic liquid crystal display elements are predominantly used in general liquid crystal display elements, and concrete examples of the nematic liquid crystal display elements actually used include: (1) a TN (Twisted Nematic) liquid crystal display element, in which a liquid crystal alignment direction of one side substrate is twisted at a 90 degree angle relative to a liquid crystal alignment direction of the other side substrate; (2) a STN (Super Twisted Nematic) liquid crystal display element, in which a liquid crystal alignment direction of one side substrate is twisted at an angle greater than 180 degrees relative to a liquid crystal alignment direction of the other side substrate; and (3) a TFT (Thin Film Transistor) liquid crystal display element which uses a thin film transistor. In the nematic liquid crystal display elements, the liquid crystal alignment is primarily controlled by a liquid crystal alignment film, and a pretilt angle of the liquid crystal relative to the liquid crystal alignment film is formed.
A liquid crystal alignment film is made from a liquid crystal alignment agent. Conventionally, the liquid crystal alignment agent is formulated by dissolving polyamic acid or polyimide in an organic solvent, and is then applied on a transparent conductive film formed on a substrate followed by baking treatment and alignment treatment to form the liquid crystal alignment film.
There is increasing demand for the displaying qualities of the liquid crystal display device. In order to develop a liquid crystal display device having improved performance, such as good response speed, high voltage holding ratio, and high reliability, JP 2-282726 discloses a liquid crystal alignment agent made of a specific polyamic acid compound and a liquid crystal alignment film made from the liquid crystal alignment agent, and JP 3-179323 discloses a liquid crystal alignment film formed from a liquid crystal alignment agent made of a specific polyamic acid compound. The liquid crystal alignment films disclosed in the prior art have high pretilt angle, and the liquid crystal display devices including the liquid crystal alignment films have improved response speed and reliability.
However, the requirement for the performance of the liquid crystal display device is growing more stringent recently because of the wide application of the liquid crystal display device of transmission type, reflection type, and transflection type, and because of a tendency toward the large-sized substrate, device, or production line. Although response speed, voltage holding ratio, and reliability are enhanced in the liquid crystal alignment films disclosed in the aforesaid prior art, there are still problems such as inferior pretilt angle stability and large variation of the pretilt angle in a liquid crystal alignment film, which result in disadvantage of defect (mura) appearance. Furthermore, there is also a problem of process instability, which results in a high defective rate.